Control circuit for electromagnetic devices



Feb. 13, 1962 R. A. PICKENS 3,021,454

CONTROL CIRCUIT FOR ELECTROMAGNETIC DEVICES Filed April 11, 1958 L SOLENOID m I-FIG.

R2 TX2 E 2 I, 02 3 c4 w TX3 L SOLENOID ROBERT A. PICKENS FIG. 5 INVENTOR United States Patent Ofiice 7 3,021,454 Patented Feb. 13, 1962 3,021,454 CONTROL CIRCUIT FOR ELECTRO- MAGNETIC DEVICES Robert A. Pickens, Baltimore, Md., assignor to The Bendix Corporation, a corporation of Delaware Filed Apr. 11, 1958, Ser. No. 727,986 6 Claims. (Cl. 317-1485) This invention relates to arrangements for accurately controlling the activation of electromagnetic devices. Although not restricted thereto, it is particularly advantageous for use in connection with the solenoid actuated valves of a fuel injection system for internal combustion engines of the type disclosed in US. patent application Serial No. 637,852, filed February 4, 1957 in the names of Robert W. Sutton, Stephen G. Woodward and Curtis A. Hartman and entitled Fuel Injection System, and now Patent No. 2,980,090.

In such a system each cylinder of the engine is pro-, vided with a solenoid actuated valve, normally closed, to which fuel is constantly supplied under pressure. The

solenoids are sequentially energized, to open the valves,

by means of rectangular voltage impulses derived from a multivibrator or other pulse forming circuit. The multivibrator is triggered in synchronism with engine rotation and its output pulses are applied sequentially to the valves for the various cylinders by a distributor likewise driven in synchronism with engine rotation.

In some instances the output impulses of the multivibrator are of sufficient power to activate the valve sole noids directly. In many cases, however, such multivibrator output power is not feasible and there must be employed a switching mechanism actuated by the output of the multivibrator or by some other means operated in synchronism with engine rotation, to apply power from another source to the solenoids in the proper sequence as dictated by the distributor.

This switching means can be made electronic, which is desirable, by the use of a transistor circuit. In switching circuits of this type known to the art, however, the total amount of the solenoid energizing current must pass through the transistor. 'This requires the use of anexpensive, power-type transistor for this service.

It is an object of the present invention to provide a transistorized switching circuit capable of effecting the energization of a solenoid from a power source for the duration of a selected time interval by the passage of the energizing current through the transistors for only a small portion of the interval.

It is another object of the invention to provide such a circuit which will be less expensive thanswitching circuits requiring power-type transistors.

These, and other objects and advantages of the invention are realized in a circuit in which the solenoid is constantly subjected to a biasing current from the power source through a resistor, which current is not sufficient to move the valve to its activated position but is sufiicient to maintain it in that position once it has been activated by a higher surge of current. An energy storage element such as a capacitor is connected to the junction of said solenoid and resistor and connected to one terminal of the power source through a'transistor switch in parallel with said resistor and to the other terminal through a second transistor switch in parallel with said solenoid. The output of a multivibrator or other squarewave source is applied by differentiating and rectifying networks to said transistor switches.

In the drawing:

FIG. 1 is a schematic'diagram of a circuit of the type normally used to operate a solenoid actuated valve or similar device utilizing a mechanical switch;

FIG. 2 is a schematic diagram of a similar circuit in which the'switch has been replaced by a power transistor switched by a squarewave input;

FIG. 3 is a schematic diagram of a circuit employing principles utilized in the present invention, butusing mechanical switches;

FIG. 4 is a graph of a voltage waveform of the type produced by the operation of the circuit of FIG. 3; and

FIG. 5 is a schematic diagram of a circuit embodying the invention.

Referring now more particularly to the drawing, there is shown in FIG. 1 an electromagnet energizing circuit comprising a solenoid L, a resistor R1, a battery E and a single pole, single throw switch S1 all connected in series. By closing the switch the solenoid is energized and by opening it the solenoid is deenergized. It is understood that the solenoid activates a core 6 which may be a valve stem, the solenoid and its core constituting a bar electromagnet.

In the circuit of FIG. 2 the switch S1 has been replaced by a transistor TX1 which is shown a being of the PN? type. The battery E is part of a series circuit including solenoid L, resistor R1, the collector and emitter of transistor TX1, and a resistor R2. *The base of TX1 is connected to an input terminal 2 and through a resistor R3 to the positive terminal of the power source E.

The resistors R1, R2 and R3 have values so selected that the transistor is normally non-conducting, thus actingas an open switch. Upon the. application to the terminal 2 of a negative-going squarewave as shown at 3, the transistor becomes conductive and remains so for the duration of the impulse 3. This amounts to a closfor the total time.

It is evident that all current necessary for lifting the core of the electromagnet must be supplied by transistor TX1 and that this level of current persists throughout the duration of the impulse3. This requires a power-type transistor at TX1 and entails all the problems of size, cost and heat dissipation which occur in connection with transistors which -can handle high current and dissipate large amounts of power.

It is a characteristic of solenoid actuated devices, including valves, that the minimum current which will maintain the device in an activated state, or hold the valve open, if it is a valve, is considerably less than the minimum current which will open the valve or actuate the device from the closed or inactive position. This fact is made use of in connection with the invention.

The'minimum hold-open current may be designated at 1, and the minimum opening current as I If a bias current 1 such that I I I is caused to flow continuous-ly through the solenoid winding the valve will remain in either its open or closed position until the total solenoid current is modified. 1 7

Assume that the valve is normally closed, and that bias current 1, is present. If a positive current pulse I (l' I is applied to the solenoid, the valve will open and remain open after the termination of current pulse 1,, since bias current I is still present. Should a negative current pulse I (I --I be applied, the valve will close and will remain closed since I I FIG. 4 illustrates the desired current waveforms. The. effective valve open time is the period t t,,.

FIG. 3 shows a circuit using mechanical switches which is capable of providing the waveform of FIG. 4. The solenoid L is connected in series with resistor R2 and the source E. A capacitor CZ is connected across re sistor R2 through a single pole, single throw switch S2. The capacitor is also connected across the solenoid L through a second switch S3.

In the operation of the circuit of FIG. 3, the bias current L is supplied by the electromotive force established by B through R2. If the switch S2 is closed, the charging current through capacitor C2 will serve as the current pulse I to activate the valve. Switch S2 may be opened at any time after I, becomes equal to or greater than M, since the solenoid will remain energized due to the presence of the bias current L and the valve will remain in its activated position.

The condenser C2 will hold its charge until switch S3 is closed, whereupon the discharging current will form pulse 1,, which will partially deenergize the solenoid. Switch S3 may be opened at any time after 1 becomes equal to or less than I since the valve will then have returned to'its rest position. 7

FIG. illustrates a transistorized version of the circuit of FIG. 3. The capacitor C2 is connected in series with transistor TX2 and R iacross resistor R2 and in series with transistor TX3 and R5 across the solenoid. Transistor TX2 is of the PNP type and TX3 is of the NPN type. The input terminal 2 is connected to the base of TX2 through a capacitor C3 and a diode D1 in series. It is also connected to the base of TX3 through a capacitor C4 and a diode D2, in series. The terminals of D1 are connected by resistors R8 and R6, respectively, to the positive terminal of the source E, while the terminals of D2 are likewise connected by resistors R9 and R7, respectively, to the negative terminal of the source.

Upon the application of the square pulse 3 to the terminal 2 it is diiterentiated by the capacitor C3 and R8 to form the waveform shown at 4, which is applied to the diode D1. The diode passes only the negative spike which coincides in time with the leading edge of the pulse 3. The transistors TX2 and TX3 are normally non-conductive due to the voltages from the source E to which they are subjected. The negative spike renders TX2 conductive long enough to charge C2 with the current pulse I,,. The current flow 1,, through the electromagnet incident to the charging of the capacitor C2 is in aiding relationship to the biasing current 1, such that the sum of said currents is sufficient to activate the electromagnet core. At the termination of the negative spike the transistor again becomes non-conductive and the current through the electromagnet drops to the I value which maintains the electromagnet core in its activated position.

The elements C4 and R9 also differentiate the waveform 3 into a waveform 5 identical with 4. This is applied to the diode D2 which is so poled as to pass only the positive spike which coincides in time with the trailing edge of the pulse 3. This renders the transistor TX3 conductive for its duration, discharging C2 to provide the current pulse 1 The current flow l through the electromagnet incident to the discharge of the capacitor is in an opposing relationship to the biasing current 1 such that the sum of said currents is less than I and thus is insutficient to maintain the electromagnet core in an activated position.

The duration of the spikes of the waveforms 4 and 5 is determined by the values of the RC elements R8, R9, C3 and C4. This also governs the duration of the conductive intervals of: the transistors. The amount of charging and discharging current (the magnitude of the pulses 1,, 1b), is controlled by the values of resistors R4, R5, R6, and R7.

I claim:

1. A circuit for activating an electromagnet having a core movable from a position of rest to an activated positon, comprising: a source of electromotive force, means including a resistor connecting said electromagnet in series with said source in asense such as to establish a biasing current flow through said electromagnet tending to activate the core thereof, said resistor having such resistance that said biasing current is insufficient to move said core to its activated position but is sufiicient to maintain said core in said activated position, a capacitor, means for applying voltage impulses to said circuit, means responsive to a voltage impulse of a first sense to momentarily establish a first conductive path connecting said capacitor in charging relation in series with said source and said electromagnet in a sense such that current flows through said electromagnet incident to the charging of said capacitor is in aiding relationship to said biasing current, the resistance of said path being such that the sum of said biasing and charging currents is sufficient to activate said electromagnet core to said activated position, means responsive to a voltage impulse of a second sense to momentarily establish a second conductive path connecting said capacitor in discharging relation in series with said electromagnet in a sense such that current flow through said electromagnet incident to the discharging of said capaictor is in opposing relationship to said biasing current thereby reducing the current flow through said electromagnet to a value which is insufficient to maintain the activated position of said electromagnet core.

2. A circuit for activating an electromagnet having a core movable from a position of rest to an activated position, comprising: a source of electromotive force, means including a resistor connecting said electromagnet in series with said source in a sense such as to establish a biasing current flow through said electromagnet tending to activate the core thereof, said resistor having such resistance that said biasing current is insufficient to move said core to its activated position, a capacitor, means responsive to a vonage impulse of a first sense to momentarily establish a first conductive path comiecting said capacitor in charging relation in series with said source and said electromagnet in a sense such that current flow through said electromagnet incident to the charging of said capacitor is in aiding relationship to said biasing current, the resistance of said path being such that the sum of said biasing and charging currents is sufficient to activate said electromagnet core to said activated position, means responsive to a voltage impulse of a second sense to momentarily establish a second conductive path connecting said capacitor in discharging relation in series with said electromagnet in a sense such that current flow through said electromagnet incident to the discharging of said capacitor is in opposing relationship to said biasing current thereby reduc ing the current flow through said electromagnet to a value which is insufiicient to maintain the activated position of said electromagnet core, means for applying a rectangular voltage impulse to said circuit, means differentiating said rectangular voltage impulse to derive voltage impulses of opposite sense from the leading and trailing edges thereof, means applying the voltage impulse coincident With said leading edge to said means establishing said first path and means applying said voltage impulse coincident with said trailing edge to said means estahilshing said second path.

3. A circuit for controlling the operation of a valve which is biased toward and normally occupies a first position and is movable by the energization of a solenoid to a second position, said circuit comprising: a source of electro-motive force, means including a resistor connecting said solenoid in series with said source in a sense such as to establish a biasing current flow through said solenoid tending to move said valve to said second position, a capacitor, means including a switch connecting said capacitor across said resistor, said switch normally eing open and being responsive to a voltage impulse of a first polarity to close, said last named means connecting said source to simultaneously charge said capacitor and to provide sufficient energizing current through said said solenoid that said valve moves to said first position,

and means sequentially applying voltage pulses of said first and second polarity to said switches.

4. A circuit for controlling the operation of a valve which is biased toward and normally occupies a first position and is movable by the energization of a solenoid to a second position, said circuit comprising: a source of elcctro-motive force, means including a'resistcr connecting said solenoid in series with said source in a' sense such as to establish a biasing current flow through said solenoid tending to move said valve to said second position, a capacitor, means including a switch connecting said capacitor across said resistor, said switch nr-- mally being open and being responsive to a voltage impulse of a first polarity to close, said last named means connecting said source to simultaneously charge said capacitor and to provide suflicient energizing current through said solenoid to move said valve to said second position, and other means including a second switch connecting said capacitor across said solenoid, said second switch normally being open and being responsive to a voltage impulse of a second polarity to close, said other means connecting said capacitor to discharge and simultaneously provide sufiicient drop in the biasing current through said solenoid that said valve moves to said first position, and means sequentially applying voltage pulses of said first and second polarity to said switches, the last named means comprising means for applying a rectangular voltage wave to said circuit, means deriving from the leading edge of said wave a voltage impulse of said first polarity and means deriving from the trailing edge of said wave a voltage impulse of said second polarity.

5. A circuit for controlling the operation of a valve said solenoid to move said valve to said second position, and other means including a second switch connecting said capacitor across said solenoid, said second switch normally being open and being responsive to a voltage impulse of a second polarity to close, said other means connecting said capacitorto discharge and simultaneously provide suflicient drop in the biasing current through said solenoid that said valve moves to said first position, said first and second switches comprising a first and second transistor respectively, means connecting said transistors to said source in a manner to render them non-conductive, means applying a rectangular voltage wave to said circuit, means deriving from the leading and trailing edges of said wave voltage impulses of said first and second polarity respectively, and means applying said impulses respectively to said transistors in a manner to render said respective transistors conductive for the duration thereof.

6. A circuit for energizing an electromagnet having a core movable from a position of rest to an activated a i 7 position, comprising: a source of electro-motive force, a means including a resistor connecting said electromagnet in series with said source, said'resistor having such resistance that the current flowing through said electromagnet is insufficient to move said core to its activated position but is sufficient to maintain said core in said activated position, a capacitor, means including a first switch connecting said capacitor across said resistor,

means including a second switch connecting said capacitor across said electromagnet, means for applying a rectangular voltage impulse to the circuit, means responsive to the leading edge of said rectangular voltage imwhich is biased toward and normally occupies a first poly being open and being responsive "to a voltage impulse of a first polarity to close, said last named means connecting said source to simultaneously charge said capacitor and to provide suflicient energizing current through pulse to momentarily close said first switch to activate said electromagnet core to the activated position and to simultaneously charge said capacitor through said elec- References Cited in the file of this patent UNITED STATES PATENTS Lorenz Sept. 25, 1956 2,904,726 Ricketts Sept. 15, 1959 a FOREIGN PATENTS 481,054 Great Britain Mar. 4, 1933 

